1. Field of the Invention
This invention relates to waste management and more particularly to the volume reduction of wastes. In one of its more particular aspects this invention relates to a process for reducing the volume of low-level radioactive wastes. In another of its more particular aspects, this invention relates to a process for producing dry, flowable solids from liquid wastes.
2. Prior Art
Waste management frequently involves the necessity of disposing of large volumes of materials, some of which may be contaminated with hazardous substances. In nuclear power plants, for example, large amounts of radioactive liquid and solid wastes, known as low-level radioactive wastes, are produced. Low-level radioactive wastes differ from high-level radioactive wastes, which are produced in the reprocessing of nuclear fuels, in that the latter present greater risks of contamination and therefore require disposal techniques which are more stringent than in the case of low-level radioactive wastes. Disposal of radioactive wastes in general cannot be readily accomplished by using conventional waste disposal techniques. Because of the relatively long half-lives of certain radioactive elements, the most widely used disposal techniques are storage, solidification and burial. The expense of so disposing of large volumes of radioactive wastes, however, is constantly rising and approaching levels at which volume reduction becomes economically desirable.
Many efforts have been directed at reducing the volume of radioactive wastes.
U.S. Pat. No. 3,101,258 describes a heated-wall spray calcination reactor useful for disposing of nuclear reactor waste solutions. In spray calcination reactors of the heated-wall type, however, the temperature gradient from the outside of the reactor inward may result in uneven heating, producing regions of undesired high temperatures and causing non-uniform results.
U.S. Pat. No. 3,922,974 discloses a hot air-fired furnace for incinerating radioactive wastes. The use of this apparatus, however, results in the production of noxious off-gases which require additional processing for removal.
U.S. Pat. No. 4,145,396 describes a process for reducing the volume of organic waste material contaminated with at least one volatile compound-forming radioactive element selected from the group consisting of strontium, cesium, iodine and ruthenium. The selected element is fixed in an inert salt by introducing the organic waste and a source of oxygen into a molten salt bath maintained at an elevated temperature to produce solid and gaseous reaction products. The molten salt bath comprises one or more alkali metal carbonates and may optionally include from 1 to about 25 wt. % of an alkali metal sulfate. Although effective to some extent in reducing the volume of organic wastes, further volume reduction involving the separation of the radioactive materials from the non-radioactive components of the molten salt bath requires a number of additional processing steps.
In U.S. patent application Ser. No. 451,516, filed Dec. 20, 1982 and assigned to the assignee of the present invention, there is proposed a process for converting radioactive wastes in the form of liquids, solids and slurries into a mixture of a non-radioactive gas and a radioactive inorganic ash. In accordance with that process the radioactive waste is introduced as a finely atomized spray into a zone heated by means of a hot gas to a temperature sufficient to effect the desired conversion, preferably a temperature in the range of about 600.degree. to 850.degree. C. The process is conducted in a spray dryer modified to combust or calcine the waste.
While the foregoing patent application discloses a process which is satisfactory for destroying radioactive wastes, the high temperatures utilized in the process can produce noxious gases such as NO.sub.x or SO.sub.x, the removal of which necessitates taking additional measures to ensure that any gas ultimately released to the atmosphere is non-polluting. In addition, such high temperatures may cause the volatilization of radionuclides from the radioactive waste.
Consequently, there is a need for a process which can be used to reduce the volume of radioactive wastes without producing noxious off-gases or volatilizing radionuclides. This need is particularly pronounced in the case of liquid low-level radioactive wastes where large volumes of wastes of relatively low radioactivity compound the problems and costs involved in their transportation and disposal.